Papers used for printing or writing usually contain, as fillers, inorganic particles of silica, hydrous silicic acid, talc, calcium carbonate, clay, kaolin, titanium dioxide, etc. and organic particles of urea/formaldehyde polymers or the like for improving the optical properties such as opacity and brightness, smoothness, touch, printability, writing suitability, etc.
Papers containing the above-described fillers are produced by adding the fillers and other assistants usually used for making paper, to paper pulp dispersed in water, forming a wet paper from the obtained stuff with a Fourdrinier paper making machine, twin-wire paper-making machine or the like and drying it.
Recently, the thickness of the paper sheets tends to be reduced to reduce the basis weight thereof However, particularly when printing paper sheets are reduced in weight, the opacity of the printed paper (hereinafter referred to as "opacity-after-printing") is reduced to cause a problem that the printed letters on the reverse side of the paper sheets are seen through the paper from the surface of the paper.
Various fillers are usually added to the papers for the purpose of improving the opacity (including the opacity-after-printing) of the papers.
Although investigations have been conducted for the purpose of developing inorganic and organic fillers for improving the opacity, inexpensive fillers having an excellent effect of improving the opacity have not yet been developed. Further, since the tendency to the further reduction in weight is recently increasing, the development of a filler having a higher power of improving the opacity than that of ordinary fillers is eagerly demanded.
Among the fillers currently used for improving the opacity, titanium dioxide has only a low power of inhibiting the penetration of inks, while it is capable of improving the opacity of white papers. Therefore, the improvement in opacity-after-printing is impossible when titanium dioxide is used. Further, under such conditions that the maximum light scattering capacity can be exhibited, the retention of titanium oxide in the paper is uneconomically very low.
Although organic urea/formalin resins have effects of improving both opacity-after-printing and opacity of white paper (opacity before printing), each absolute effect is insufficient.
Hydrous silicic acid is less expensive than the other fillers and it is relatively effective in imparting the opacity-after-printing to a paper by inhibiting the penetration of inks when it is added to a pulp and the paper is made therefrom. However, its effects including that of improving the opacity of white paper have not yet reached the expected level.
As for hydrous silicic acid, it is known that the oil absorption, which is an index of the capacity of preventing ink from the penetration and greatly contributes to the improvement in the opacity-after-printing, is increased in proportion to the specific surface area of hydrous silicic acid and can be controlled by changing the hydrous silicic acid synthesis conditions.
However, when the specific surface area of hydrous silicic acid is increased to a level higher than that of the ordinary one and the paper is dried under the same conditions as the paper-making conditions, the filler itself is shrunk to reduce the oil absorption and, as a result, the improvement in the opacity-after-printing is reduced.
Amorphous silica particles disclosed in Japanese Patent No. 2,604,316 have a high oil absorption and not so large specific surface area. However, the effect thereof on the opacity obtained by adding these particles to paper is not significantly different from that by adding hydrous silicic acid commonly used at present.
Japanese Patent Unexamined Published Application (hereinafter referred to as "J. P. KOKAI") No. Hei 5-301707 discloses hydrous silicic acid of the formula: SiO.sub.2.multidot.nH.sub.2 O (wherein n is a positive integer) wherein the cumulative volume is 1.9 to 4.0 cc/g for pores having a pore radius in the range of 5.times.10.sup.4 .ANG. or less, that is at least 0.5 cc/g for pores having a pore radius in the range of 3,000 to 4.times.10.sup.4 .ANG., and that is at least 0.6 cc/g for pores having a pore radius in the range of 100 to 1,000 .ANG.. However, the absolute volume for the pores is small because the cumulative volume of pores having a pore radius of 5.times.10.sup.4 .ANG. or smaller is up to 4.0 cc/g. In addition, the cumulative volume of the pores having a pore radius of 3,000 to 4.times.10.sup.4 .ANG. and capable of absorbing the pigment in the ink and also that of the pores having a pore radius of 100 to 1,000 .ANG. and capable of absorbing the vehicle in the ink are yet below the necessary levels.
Japanese Patent No. 2,710,529 discloses a hydrous silicic acid filler for paper making, which is fine, amorphous hydrous silicic acid obtained by the neutralization reaction of an aqueous sodium silicate solution in the absence of alkali metal salts, and which contains at least amorphous magnesium silicate as a fine, amorphous metal compound. However, when the amorphous metal compound content is increased for improving the opacity, the oil absorption is reduced to impair the power of inhibiting the penetration of the ink and, as a result, the improvement in the opacity-after-printing is unsatisfactory.
The diameters of primary particles usually and widely used are very small. Although the particle diameters are relatively uniform, they are not in the form of the primary particles, but they form aggregates as secondary particles in most cases, and the particle diameter distribution is usually wide when the particles are used. In addition, even when the average particle size is almost equal, the state of the distribution is different. It is known that when particles of a small diameter are thoroughly dispersed in a paper, the contribution of the particles on the optical properties of the paper is greater than that of particles of a large diameter or a small diameter retained in the paper with a reagent such as a retaining improver.
However, the particle size of the filler used ranges in a wide range as described above. When such a filler of various particle sizes is added to a paper-making pulp and a paper is made therefrom, the retaining rate (or retention) of the particles of small sizes is usually and seriously low, though it varies depending on the paper-making machine. For improving the optical properties, the addition rate of the filler must be increased. However, the improvement in the optical properties by merely increasing the addition rate is limited because when the addition rate of the filler is increased, the strength of the paper is lowered. On the other hand, although the particles of a large size are retained in the paper, they also have problems that the specific surface area of them is small and the contribution of them to the optical properties is only slight. Under these circumstances, it is desired to easily obtain a filler having a uniform particle size.